Help with double lav code

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Snoqualmie, WA
I have a couple of code related questions for a double lav. as I cannot get a hold of my plumbing inspector (Seattle).

For the 4 way fitting where the two drains converge it seems like there’s a couple of options.. Double Sanitary tee seems to be out of the question, more or less... but what about these options:

-double wye, with a street 45 on both ends.

-double fixture fitting 2” all the way around. Ideally I want to use a double fixture 2x2x1.5x1.5 but this fitting is $35 at the supply house (not at Home Depot).
If I went this route should I just use 2” for the drains also, or use a 1.5” hub adapter for the drain?

Is a 1.5” vent for two lav sufficient or is 2” that much better it’s worth it to do 2”?

Is there a maximum distance a drain can run horizontal before reaching the 4way fitting? There won’t be any venting until I reach the 4way fitting... unless it’s needed?

a sweeping 90 here for the drain-stubout as it’s a horizontal transition?

Thank you for the help! Just a basic sketch so it’s clear what I’m talking about


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In the Trades
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Berkeley, CA
The relevant points for the UPC follow. I'm assuming you want to run your lav drains horizontal until they meet at the location where they jointly turn down, rather than, say, turning each one down earlier and combining them some other way.

- The trap arm is the drain starting at the trap outlet up until the trap is vented. For a 1-1/4" trap, the length of the trap arm is limited to 30". For a 1-1/2" trap, it is limited to 42". The trap arm needs to fall at least 1/4" per foot, but may not fall more than one trap diameter. The trap arm in the wall is typically 1-1/2", even if there's a 1-1/4" trap outside the wall. Using a 1-1/2" tubular trap with a 1-1/4" x 1-1/2" reducing SJ washer to receive the lav tailpiece is a good approach.

- If you want the "4-way fitting" to vent both traps, and that configuration works as the trap arms will be short enough, then you must use a double fixture fitting, a double combo (wye plus 45) is not allowed. You could also use stacked san-tees (with a 2" vertical drain between them), but then your trap arms would be at different heights.

- If 42" is insufficient for your trap arm length, then you must separately vent the trap arm. To do that you use a 1-1/2" upright combo, where the straight path is horizontal, the side path is pointing up for your vent, and water running down the vent would be directed downstream by the curve of the combo. If both trap arms are separately vented, then you can use a double combo as your "4-way fitting". Although if the lavs aren't wet venting anything, you don't actually need the top (vent) connection on that double combo when both lavs are separately vented, you could just plug it.

- A 1-1/2" vent is sufficient for the lavs, but you need to know what's happening downstream. If the lavs go on to wet vent a WC, that requires a 2" vent, so the vent on the 4-way fitting should be 2". The common vertical drain should be 2".

- Yes, the lav drain stubout should be a long sweep 90 as is it's a horizontal to horizontal transition. However, I understand that it's common to use a quarter bend as the curve sticks out of the wall less, and that this is overlooked.

- If you want a 2x2x1.5x1.5 fitting, and all you can get is a 2x2x2x2 fitting, then using 2x1.5 bushings is a good approach.

Cheers, Wayne

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